The present invention claims the benefit of Korean Patent Application No. 2001-87390 filed in Korea on Dec. 28, 2001, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to an active matrix organic electroluminescence display device including an organic emissive layer.
2. Discussion of the Related Art
A cathode ray tube is widely used as a display device such as a television and a monitor, and the cathode ray tube has a large size, heavy weight, and high driving voltage. Therefore, flat panel displays, which have properties of being thin, low weight and low power consumption, have been proposed. The flat panel displays include a liquid crystal display device, a plasma display panel, a field emission display device, and an electroluminescence display device.
Electroluminescence display devices may be categorized into inorganic electroluminescence display devices and organic electroluminescence display devices according to source material for exciting carriers. The organic electroluminescence display device has attracted considerable attention lately due to its high brightness, low driving voltage, and natural color images from all colors of a visible light spectrum. Additionally, the organic electroluminescence display device has great contrast ratio because of self-luminescence. The organic electroluminescence display device can easily display moving images due to short response time of several microseconds, and is not limited by a viewing angle. The organic electroluminescence display device is stable at a low temperature, and its driving circuit can be fabricated easily because it is driven by low voltage. Besides, manufacturing process of the organic electroluminescence display device is relatively simple.
In general, an organic electroluminescence display device emits light by injecting an electron from a cathode electrode and a hole from an anode electrode into an emissive layer, combining the electron with the hole, which generates an exciton, and transiting the exciton from an excited state to a ground state.
Because of its luminous mechanism similar to a light emitting diode, the organic electroluminescence display device may be called an organic light emitting diode (OLED).
Organic electroluminescence display devices are classified into a passive matrix type and an active matrix type according to a driving method.
The passive matrix organic electroluminescence display device has a simple structure and is manufactured through a simple process. However, the passive matrix organic electroluminescence display device has high power consumption and is difficult to manufacture to have a large area. Additionally, aperture ratio in the passive matrix organic electroluminescence display device decreases according to the increasing number of electro lines.
Therefore, the passive matrix organic electroluminescence display device is widely used as a small size display device. On the other hand, the active matrix organic electroluminescence display (AMOELD) device is widely used as a large size display device.
An active matrix organic electroluminescence display (AMOELD) device according to the related art will be described hereinafter more in detail.
FIG. 1 is an equivalent circuit diagram for a pixel of an AMOELD device in the related art. In FIG. 1, a pixel of an AMOELD device includes a switching thin film transistor (TFT) 4, a driving thin film transistor (TFT) 5, a storage capacitor 6, and an electroluminescent diode 7.
A gate electrode of the switching TFT 4 is electrically connected to a gate line 1, and a source electrode of the switching TFT 4 is electrically connected to a data line 2. A drain electrode of the switching TFT 4 is electrically connected to a gate electrode of the driving TFT 5. A drain electrode of the driving TFT 5 is electrically connected to an anode electrode of the electroluminescent diode 7, and a source, electrode of the driving TFT 5 is electrically connected to a power line 3. A cathode electrode of the electroluminescent diode 7 is grounded. The storage capacitor 6 is electrically connected to the gate electrode and the source electrode of the driving TFT 5.
When a signal is applied to the gate electrode of the switching TFT 4 through the gate line 1, the switching TFT 4 turns on. At this time, a signal from the data line 2 is transmitted to the gate electrode of the driving TFT 5 through the switching TFT 4 and is stored in the storage capacitor 6. Then, the driving TFT 5 is turned on by the signal from the data line 2, and a signal from the power line 3 is transmitted to the electroluminescent diode 7 through the driving TFT 5. Therefore, light is emitted from the electroluminescent diode 7. Brightness of the device of FIG. 1 is regulated by controlling current passing through the electroluminescent diode 7.
Here, even though the switching TFT 4 turns off, the driving TFT 5 maintains in an xe2x80x9con statexe2x80x9d because of the signal stored in the storage capacitor 6. Accordingly, light is emitted by current continuously passing through the electroluminescent diode 7 until the next signal is transmitted to the gate electrode of the driving TFT 5 through the switching TFT 4.
FIG. 2 illustrates a plan view for the pixel of an AMOELD device in the related art. In FIG. 2, a gate line 41 and a data line 61 cross each other and define a pixel region xe2x80x9cPxe2x80x9d. A power line 67 is formed parallel to the data line 61.
A switching TFT xe2x80x9cTSxe2x80x9d is formed at the crossing of the gate line 41 and the data line 61 and is connected to the gate line 41 and the data line 61. A driving TFT xe2x80x9cTDxe2x80x9d is formed in the pixel region xe2x80x9cPxe2x80x9d and is connected to the switching TFT xe2x80x9cTSxe2x80x9d.
As stated above, the gate electrode 42 of the driving TFT xe2x80x9cTDxe2x80x9d is connected to the drain electrode of the switching TFT xe2x80x9cTSxe2x80x9d and a first capacitor electrode 45 of the storage capacitor. The source electrode 62 of the driving TFT xe2x80x9cTDxe2x80x9d is connected to the source region 22 through a first contact hole 50a. The source electrode 62 is also connected to the second capacitor electrode 65 of the storage capacitor, and the second capacitor electrode 65 is connected to the power line 67. The second capacitor electrode 65 forms the storage capacitor with the overlapped first capacitor electrode 45. A drain region 23 of the driving TFT xe2x80x9cTDxe2x80x9d overlaps a pixel electrode 81 formed in the pixel region xe2x80x9cPxe2x80x9d, and the drain region 23 is connected to the pixel electrode 81 through a second contact hole 50b. 
FIG. 3 illustrates a cross-section along the line IIIxe2x80x94III of FIG. 2. In FIG. 3, polycrystalline silicon layers 21, 22 and 23 having an island shape are formed on a substrate 10, and the polycrystalline silicon layers are divided into an active layer 21 and source and drain regions 22 and 23. The source and drain regions 22 and 23 are doped.
A gate insulator 30 is formed on the polycrystalline silicon layers 21, 22 and 23, and a gate electrode 42 and a first capacitor electrode 45 are formed on the gate insulator 30. The gate electrode 42 is disposed over the active layer 21.
An interlayer dielectric 50 is formed on the gate electrode 42 and the first capacitor electrode 45, and the interlayer dielectric 50 has first and second contact holes 50a and 50b, which also pass through the gate insulator 30, to expose the source and drain regions 22 and 23, respectively.
A source electrode 62 and a second capacitor electrode 65 are formed on the interlayer dielectric 50. The source electrode 62 and the second capacitor electrode 65 are made of a conductive material such as metal. The source electrode 62 is connected to the source region 22 through the first contact hole 50a. The second capacitor electrode 65 contacts the source electrode 62, and overlaps the first capacitor electrode 45 to form a storage capacitor.
Next, a passivation layer 70 is formed on the source electrode 62 and the second capacitor electrode 65. The passivation layer 70 has a third contact hole 71 over the second contact hole 50b. 
A pixel electrode 81 is formed on the passivation layer 70, and the pixel electrode 81 is connected to the drain region 23 through the third and second contact holes 71 and 50b. The pixel electrode 81 is an anode electrode of an electroluminescent diode and is made of a transparent conductive material.
Although not shown in the figure, an emissive layer and a cathode electrode of the electroluminescent diode are subsequently formed on the pixel electrode 81 in the AMOELD device. An opaque material is widely used as the cathode electrode. Therefore, as shown in FIG. 4, the AMOELD device in the related art is a bottom emission mode. At this time, a plurality of pixels 92 are formed in an image area 91 on an upper side of a substrate 90, which corresponds to the substrate 10 of FIG. 3. Here, an aperture ratio of the device is decreased because light passes through the only region that does not have the thin film transistors, that is, the switching TFT and the driving TFT, and the storage capacitor.
FIG. 5 shows a region through which light passes and corresponds to the region xe2x80x9cAxe2x80x9d of FIG. 4. In FIG. 5, light is emitted in the only the hatched region xe2x80x9cLxe2x80x9d of the pixel region xe2x80x9cPxe2x80x9d. Therefore, image quality is reduced because of the low aperture ratio, and more current is required in order to display bright images. The lifetime of the device may become short because of the application of increased current.
Accordingly, the present invention is directed to an active matrix organic electroluminescence display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide an active matrix organic electroluminescence display device that has a high aperture ratio and long lifetime.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. These and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, an active matrix organic electroluminescence display device includes a substrate, a gate line on the substrate, a data line crossing the gate line, a power line on the substrate, a first switching thin film transistor electrically connected to the gate line and the data line, a first driving thin film transistor electrically connected to the first switching thin film transistor and the power line, a storage capacitor electrically connected to the first switching thin film transistor, the first driving thin film transistor and the power line, and an electroluminescent diode electrically connected to the first driving thin film transistor, wherein the electroluminescent diode includes a first electrode and a second electrode over the first electrode, and the first electrode covers the storage capacitor.
The active matrix organic electroluminescence display device may further include a second switching thin film transistor and a second driving thin film transistor, wherein the second switching thin film transistor is electrically connected to the gate line, the data line and the first switching thin film transistor, and the second driving thin film transistor is electrically connected to the first switching thin film transistor, the first driving thin film transistor and the storage capacitor.
In another aspect, an active matrix organic electroluminescence display device includes a substrate, a gate line on the substrate, a data line crossing the gate line, a power line on the substrate, a plurality of thin film transistors electrically connected to the gate line and the data line, a storage capacitor electrically connected to the plurality of thin film transistors and the power line, and an electroluminescent diode electrically connected to the plurality of thin film transistors, wherein the electroluminescent diode includes a first electrode and a second electrode over the first electrode, and the first electrode covers the storage capacitor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.